Semiconductor light-emitting device and method for  manufacturing same

ABSTRACT

According to one embodiment, a semiconductor light-emitting device includes an n-type semiconductor layer including a nitride semiconductor, a p-type semiconductor layer including a nitride semiconductor, a light-emitting portion and a stacked body. The light-emitting portion is provided between the n-type and p-type semiconductor layers and includes a barrier layer and a well layer. The well layer is stacked with the barrier layer. The stacked body is provided between the light-emitting portion and the n-type semiconductor layer and includes a first layer and a second layer. The second layer is stacked with the first layer. Average In composition ratio of the stacked body is higher than 0.4 times average In composition ratio of the light-emitting portion. The layer thickness t b  of the barrier layer is 10 nanometers or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2010-050391, filed on Mar. 8,2010; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductorlight-emitting device and a method for manufacturing the same.

BACKGROUND

Group III-V nitride compound semiconductors such as gallium nitride(GaN) have wide bandgap. Exploiting this feature, they are applied tolight-emitting diodes (LED) emitting ultraviolet to blue/green lightwith high brightness, and laser diodes (LD) emitting blue-violet to bluelight.

These light-emitting devices have the following structure. On a sapphiresubstrate, for instance, an n-type semiconductor layer, a light-emittinglayer including a quantum well layer and a barrier layer, and a p-typesemiconductor layer are stacked in this order.

In such semiconductor light-emitting devices, there is demand forsimultaneously achieving low driving voltage and high light emissionefficiency.

By thinning the barrier layer, the driving voltage tends to decrease.However, thinning the barrier layer results in degraded crystallinity,which decreases the light emission efficiency. On the other hand, thequantum well layer is made of e.g. InGaN. Here, nonuniform compositionratio of In and difference in lattice constant cause lattice strain inthe quantum well layer. As a result, defects are generated in thecrystal. Furthermore, the piezoelectric field induced by the strainmodulates the band structure of the active layer and decreases the lightemission efficiency.

With a view to increasing the light emission output power, decreasingthe forward voltage, and improving the electrostatic breakdown voltage,a nitride semiconductor device is described in JP-3424629. This nitridesemiconductor device includes an active layer between an n-type nitridesemiconductor layer and a p-type nitride semiconductor layer. The n-typenitride semiconductor layer includes an n-type contact layer and ann-type multi-film layer with a superlattice structure. Furthermore, anundoped GaN layer having a film thickness of 100 angstroms or more isinterposed between the n-type contact layer and the n-type multi-filmlayer. However, despite such conventional techniques, there is room forimprovement to achieve low driving voltage and high light emissionefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of part of a semiconductor light-emitting device;

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of the semiconductor light-emitting device;

FIGS. 3 to 8 are graphs illustrating an experimental result related tothe semiconductor light-emitting devices; and

FIG. 9 is a flow chart illustrating a method for manufacturing asemiconductor light-emitting device.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor light-emittingdevice includes an n-type semiconductor layer including a nitridesemiconductor, a p-type semiconductor layer including a nitridesemiconductor, a light-emitting portion and a stacked body. Thelight-emitting portion is provided between the n-type semiconductorlayer and the p-type semiconductor layer and includes a barrier layerand a well layer. The barrier layer includes In_(b)Ga_(1-b)N (0≦b<1) andhas a layer thickness t_(b) (nanometers). The well layer is stacked withthe barrier layer, includes In_(w)Ga_(1-w)N (0<w<1, b<w), and has alayer thickness t_(w) (nanometers). The stacked body is provided betweenthe light-emitting portion and the n-type semiconductor layer andincludes a first layer and a second layer. The first layer includesIn_(x)Ga_(1-x)N (0≦x<1) and has a layer thickness t_(x) (nanometers).The second layer is stacked with the first layer, includesIn_(y)Ga_(1-y)N (0<y<1, x<y<w), and has a layer thickness t_(y)(nanometers). Average In composition ratio of the stacked body is higherthan 0.4 times average In composition ratio of the light-emittingportion, assuming that the average In composition ratio of thelight-emitting portion is (w×t_(w)+b×t_(b))/(t_(w)+t_(b)), and theaverage In composition ratio of the stacked body is(x×t_(x)+y×t_(y))/(t_(x)+t_(y)). The layer thickness t_(b) of thebarrier layer is 10 nanometers or less.

According to another embodiment, a semiconductor light-emitting deviceincludes a substrate, an n-type semiconductor layer, a stacked body, alight-emitting portion and a p-type semiconductor layer. The n-typesemiconductor layer is provided on the substrate and includes a nitridesemiconductor. The stacked body is provided on the n-type semiconductorlayer and includes a first layer and a second layer. The light-emittingportion is provided on the stacked body and includes a barrier layer anda well layer. The p-type semiconductor layer is provided on thelight-emitting portion and includes a nitride semiconductor. The barrierlayer includes In_(b)Ga_(1-b)N (0≦b<1) and has a layer thickness t_(b)(nanometers). The well layer is stacked with the barrier layer, includesIn_(w)Ga_(1-w)N (0<w<1, b<w), and has a layer thickness t_(w)(nanometers). The first layer includes In_(x)Ga_(1-x)N (0≦x<1) and has alayer thickness t_(x) (nanometers). The second layer is stacked with thefirst layer, includes In_(y)Ga_(1-y)N (0<y<1, x<y<w), and has a layerthickness t_(y) (nanometers). Average In composition ratio of thestacked body is higher than 0.4 times average In composition ratio ofthe light-emitting portion, assuming that the average In compositionratio of the light-emitting portion is (w×t_(w)+b×t_(b))/(t_(w)+t_(b)),and the average In composition ratio of the stacked body is(x×t_(x)+y×t_(y))/(t_(x)+t_(y)). The layer thickness t_(b) of thebarrier layer is 10 nanometers or less.

According to another embodiment, a method for manufacturing asemiconductor light-emitting device is disclosed. The method can includeforming an n-type semiconductor layer including a nitride semiconductoron a substrate. The method can include forming a stacked body includinga first layer and a second layer on the n-type semiconductor layer. Themethod can include forming a light-emitting portion including a barrierlayer and a well layer on the stacked body. The method can includeforming a p-type semiconductor layer on the light-emitting portion. Theforming the stacked body includes forming the first layer includingIn_(x)Ga_(1-x)N (1≦x<1) with a thickness of layer thickness t_(x)nanometers on the n-type semiconductor layer and forming the secondlayer including In_(y)Ga_(1-y)N (0<y<1, x<y) with a thickness of layerthickness t_(y) nanometers on the first layer. The forming thelight-emitting portion includes forming the barrier layer includingIn_(b)Ga_(1-b)N (0≦b<1, b<w) with a layer thickness t_(b) nanometershaving a value of 10 nanometers or less on the stacked body and formingthe well layer including In_(w)Ga_(1-w)N (0<w<1, y<w) with a thicknessof layer thickness t_(w) nanometers on the barrier layer. At least oneof the forming the stacked body and the forming the light-emittingportion is performed so that average In composition ratio of the stackedbody is higher than 0.4 times average In composition ratio of thelight-emitting portion, assuming that the average In composition ratioof the stacked body is (x×t_(x)+y×t_(y))/(t_(x)+t_(y)), and the averageIn composition ratio of the light-emitting portion is(w×t_(w)+b×t_(b))/(t_(W)+t_(b)).

Embodiments of the invention will now be described with reference to thedrawings.

The drawings are schematic or conceptual. The relationship between thethickness and the width of each portion, and the size ratio between theportions, for instance, are not necessarily identical to those inreality. Furthermore, the same portion may be shown with differentdimensions or ratios depending on the figures.

In the present specification and the drawings, the same components asthose described previously with reference to earlier figures are labeledwith like reference numerals, and the detailed description thereof isomitted as appropriate.

First Embodiment

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of part of a semiconductor light-emitting device accordingto a first embodiment.

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of the semiconductor light-emitting device according tothe first embodiment.

As shown in FIG. 2, the semiconductor light-emitting device 110according to this embodiment includes an n-type semiconductor layer 20,a p-type semiconductor layer 50, a light-emitting portion 40 providedbetween the n-type semiconductor layer 20 and the p-type semiconductorlayer 50, and a stacked body 30 provided between the light-emittingportion 40 and the n-type semiconductor layer 20.

The n-type semiconductor layer 20 and the p-type semiconductor layer 50include nitride semiconductors.

The light-emitting portion 40 is e.g. an active layer. The stacked body30 is e.g. a superlattice layer.

In the semiconductor light-emitting device 110, on the major surface(e.g., C-surface) of a substrate 10 made of e.g. sapphire is provided abuffer layer 11, for instance. An undoped GaN foundation layer 21 and ann-type GaN contact layer 22, for instance, are provided on the bufferlayer 11. The n-type GaN contact layer 22 is included in the n-typesemiconductor layer 20. The GaN foundation layer 21 may be regarded asbeing included in the n-type semiconductor layer 20 for convenience.

The stacked body 30 is provided on the n-type GaN contact layer 22. Inthe stacked body 30, for instance, first layers 31 and second layers 32are alternately stacked. The detailed configuration of the first layer31 and the second layer 32 is described later.

The light-emitting portion 40 (active layer) is provided on the stackedbody 30. The light-emitting portion 40 has e.g. a multiple quantum well(MQW) structure. That is, the light-emitting portion 40 includes astructure in which a plurality of barrier layers 41 and a plurality ofwell layers 42 are alternately and repetitively stacked. The detailedconfiguration of the barrier layer 41 and the well layer 42 is describedlater.

On the light-emitting portion 40, a p-type AlGaN layer 51, a p-type e.g.Mg-doped GaN layer 52, and a p-type GaN contact layer 53 are provided inthis order. The p-type AlGaN layer 51 functions as an electron overflowprevention layer. The p-type AlGaN layer 51, the Mg-doped GaN layer 52,and the p-type GaN contact layer 53 are included in the p-typesemiconductor layer 50. Furthermore, a transparent electrode 60 isprovided on the p-type GaN contact layer 53.

A portion of the n-type GaN contact layer 22 constituting the n-typesemiconductor layer 20, and the stacked body 30, the light-emittingportion 40, and the p-type semiconductor layer 50 corresponding to theportion are removed. An n-side electrode 70 is provided on the n-typeGaN contact layer 22. The n-side electrode 70 is made of a stackedstructure of e.g. Ti/Pt/Au. On the other hand, a p-side electrode 80 isprovided on the transparent electrode 60.

Thus, the semiconductor light-emitting device 110 of this exampleaccording to this embodiment is a light-emitting diode (LED).

As shown in FIG. 1, the light-emitting portion 40 includes a barrierlayer 41 and a well layer 42 stacked with the barrier layer 41. In thisexample, the barrier layer 41 is provided in a plurality, and the welllayer 42 is provided in a plurality. Each well layer 42 is providedbetween the plurality of barrier layers 41.

The barrier layer 41 and the well layer 42 include nitridesemiconductors. The well layer 42 is made of a nitride semiconductorcontaining In. The barrier layer 41 has a larger bandgap energy than thewell layer 42.

The barrier layer 41 includes In_(b)Ga_(1-b)N (0≦b<1). The thickness ofthe barrier layer 41 is a layer thickness t_(b) (nanometers). On theother hand, the well layer 42 includes In_(x)Ga_(1-w)N (0<w<1). Thethickness of the well layer 42 is a layer thickness t_(w) (nanometers).

Here, the In composition ratio w of the well layer 42 is higher than theIn composition ratio b of the barrier layer 41, i.e., b<w. The Incomposition ratio b of the barrier layer 41 may be zero. For instance,the barrier layer 41 may be made of GaN. On the other hand, the Incomposition ratio w of the well layer 42 is higher than zero, and thewell layer 42 includes InGaN.

In the case where the barrier layer 41 contains In, the In compositionratio b of the barrier layer 41 is lower than the In composition ratio wof the well layer 42. Thus, the well layer 42 has a smaller bandgapenergy than the barrier layer 41.

The barrier layer 41 and the well layer 42 may contain a trace amount ofAl and the like.

In this example, a plurality of barrier layers 41 and a plurality ofwell layers 42 are provided. First, for simplicity of description, it isassumed that the plurality of barrier layers 41 have an equal Incomposition ratio b, and also have an equal thickness. Similarly, it isassumed that the plurality of well layers 42 have an equal Incomposition ratio w, and also have an equal thickness.

On the other hand, as shown in FIG. 1, the stacked body 30 includes afirst layer 31 and a second layer 32. The stacked body 30 has astructure in which the first layer 31 and the second layer 32 arealternately stacked. Here, the stacked body 30 only needs to be astructure in which the first layer 31 and the second layer 32 arestacked in at least one pair.

The first layer 31 includes In_(x)Ga_(1-x)N (0≦x<1). The thickness ofthe first layer 31 is a layer thickness t_(x) (nanometers). On the otherhand, the second layer 32 includes In_(y)Ga_(1-y)N (0<y≦1). Thethickness of the second layer 32 is a layer thickness t_(y)(nanometers).

In this example, a plurality of first layers 31 and a plurality ofsecond layers 32 are provided. First, for simplicity of description, itis assumed that the plurality of first layers 31 have an equal Incomposition ratio x, and also have an equal thickness. Similarly, it isassumed that the plurality of second layers 32 have an equal Incomposition ratio y, and also have an equal thickness.

Here, the In composition ratio y of the second layer 32 is higher thanthe In composition ratio x of the first layer 31, i.e., x<y. The Incomposition ratio x of the first layer 31 may be zero. For instance, thefirst layer 31 can be made of GaN. On the other hand, the In compositionratio y of the second layer 32 is higher than zero, and the second layer32 includes InGaN.

Here, the In composition ratio y of the second layer 32 is lower thanthe In composition ratio w of the well layer 42, i.e., y<w. Hence, theIn composition ratio x of the first layer 31 is also lower than the Incomposition ratio w of the well layer 42, i.e., x<w. This suppressesabsorption of light emitted from the light-emitting portion 40 in thefirst layer 31 and the second layer 32, and increases the lightextraction efficiency. That is, the light emission efficiency isincreased.

That is, the In composition ratios of the barrier layer 41, the welllayer 42, the first layer 31, and the second layer 32 described abovesatisfy b<w and x<y<w.

Furthermore, the In composition ratio b of the barrier layer 41 and theIn composition ratio x of the first layer 31 are arbitrary as long asthe “average In composition ratios” described later satisfy the relationdescribed later.

In the semiconductor light-emitting device 110 according to thisembodiment, the layer thickness t_(b) of the barrier layer 41 is as thinas 10 nanometers (nm) or less. As a result, the driving voltage of thesemiconductor light-emitting device 110 decreases to the practicallyrequired level.

On the other hand, in the semiconductor light-emitting device 110, thestacked body average In composition ratio p of the stacked body 30 ismade higher than 0.4 times the light-emitting portion average Incomposition ratio q of the light-emitting portion 40. This suppressesstrain applied to the well layer 42 and improves the crystallinity.Furthermore, this can suppress the effect of piezoelectric field andincrease the light emission efficiency. That is, low driving voltage andhigh light emission efficiency can be simultaneously achieved.

Here, the light-emitting portion average In composition ratio q of thelight-emitting portion 40 is defined as follows.

Suppose that the well layer 42 includes In_(w)Ga_(1-w)N and has a layerthickness t_(w) (nanometers), and the barrier layer 41 includesIn_(b)Ga_(1-b)N and has a layer thickness t_(b) (nanometers). Then, thelight-emitting portion average In composition ratio q is defined as(w×t_(w)+b×t_(b))/(t_(w)+t_(b)).

The stacked body average In composition ratio p of the stacked body 30is defined as follows.

Suppose that the first layer 31 includes In_(x)Ga_(1-x)N and has a layerthickness t_(x) nanometers, and the second layer 32 includesIn_(y)Ga_(1-y)N and has a layer thickness t_(y) nanometers. Then, thestacked body average In composition ratio p is defined as(x×t_(x)+y×t_(y))/(t_(x)+t_(y)).

In the semiconductor light-emitting device 110 including thelight-emitting portion 40 and the stacked body 30 as described above,the lattice strain applied to the light-emitting portion 40 is lowerthan in the case without the light-emitting portion 40 and the stackedbody 30 as described above. Thus, the semiconductor light-emittingdevice 110 achieves high light emission efficiency and low drivingvoltage.

In general, in the well layer 42 made of InGaN, due to lattice constantdifference, lattice strain is likely to occur, and hence crystal defectsare likely to occur. Furthermore, the band energy is modulated by thepiezoelectric field due to the lattice strain, and the light emissionefficiency is likely to decrease. In the case where the thickness of thebarrier layer 41 is thinned to decrease the driving voltage, thecrystallinity is likely to be degraded. Simultaneously, theaforementioned strain applied to the well layer 42 increases. Thisfurther interferes with the increase of light emission efficiency.

In contrast, in the semiconductor light-emitting device 110 according tothis embodiment, the stacked body 30 is interposed between thelight-emitting portion 40 and the n-type semiconductor layer 20. Theaverage In composition ratio p of the stacked body 30 is made higherthan 0.4 times the light-emitting portion average In composition ratio qof the light-emitting portion 40. This relaxes the aforementionedstrain. Thus, even in the case where the thickness of the barrier layer41 is thinned to decrease the driving voltage, high light emissionefficiency can be achieved.

If the average In composition ratio p of the stacked body is equal to orlower than 0.4 times the average In composition ratio q of thelight-emitting portion 40, the effect of relaxing the aforementionedstrain may not be sufficiently achieved.

Here, if the average In composition ratio p of the stacked body 30 isequal to the light-emitting portion average In composition ratio q ofthe light-emitting portion 40, light emitted from the light-emittingportion 40 toward the n-type semiconductor layer 20 is absorbed in thefirst layer 31 and the second layer 32 of the stacked body 30. Hence, inthis embodiment, the average In composition ratio p is made lower thanthe average In composition ratio q. This can suppress the aforementionedabsorption and increase the light emission efficiency.

In general, it is considered that the driving voltage can be decreasedby thinning the layer thickness t_(b) of the barrier layer 41. However,if the layer thickness t_(b) of the barrier layer 41 is thinned, thecrystallinity of the light-emitting portion 40 tends to be degraded. Forinstance, if the layer thickness t_(b) of the barrier layer 41 is 10 nmor less, the light emission efficiency may be decreased due to thedegraded crystallinity of the light-emitting portion 40.

As a consequence of experiments, the inventors have found theaforementioned condition for allowing increase in light emissionefficiency while reducing the driving voltage by decreasing the layerthickness t_(b) of the barrier layer 41 to 10 nm or less.

In the following, the experimental results which have served as a basisfor finding the aforementioned condition are described.

In these experiments, the configuration of the light-emitting portion 40(the thickness and In composition ratio of the barrier layer 41, and thethickness and In composition ratio of the well layer 42) and theconfiguration of the stacked body 30 (the thickness and In compositionratio of the first layer 31, and the thickness and In composition ratioof the second layer 32) were varied to fabricate semiconductorlight-emitting devices. The driving voltage Vf and optical output powerPo thereof were evaluated.

First Experiment

In the first experiment, samples of the semiconductor light-emittingdevice were fabricated as follows.

First, a C-surface sapphire substrate 10, for instance, was subjected toorganic cleaning and acid cleaning. The substrate 10 was introduced intothe reaction furnace of an MOCVD apparatus. On a susceptor of thereaction furnace, the substrate 10 was heated to approximately 1100° C.Thus, oxide film on the surface of the substrate 10 is removed.

Next, on the major surface (C-surface) of the substrate 10, a bufferlayer 11 was grown to a thickness of 30 nm. Furthermore, on the bufferlayer 11, an undoped GaN foundation layer 21 was grown to a thickness of3 micrometers (μm). Furthermore, on the GaN foundation layer 21, ann-type GaN contact layer 22 made of Si-doped GaN was grown to athickness of 2 μm.

Next, on the n-type GaN contact layer 22, first layers 31 made ofIn_(x)Ga_(1-x)N and second layers 32 made of In_(y)Ga_(1-y)N werealternately stacked 30 periods to form a stacked body 30.

Here, the In composition ratio x of the first layer 31 was zero, and itsthickness was 1 nm. The In composition ratio y of the second layer 32was 0.08, and its thickness was 2.5 nm.

Next, on the stacked body 30, barrier layers 41 and well layers 42 werealternately stacked 8 periods.

In this experiment, the In composition ratio b of the barrier layer 41was zero, and the In composition ratio w of the well layer 42 was 0.15.

The layer thickness of the barrier layer 41 was varied among threevalues: 5 nm, 10 nm, and 20 nm. On the other hand, the thickness of thewell layer 42 was fixed to 2.5 nm. These samples are designated assample x1, sample x2, and sample x3. That is, the thickness of thebarrier layer 41 in the samples x1, x2, and x3 is 5 nm, 10 nm, and 20nm, respectively.

On the last well layer 42, in all the samples x1, x2, and x3, a barrierlayer 41 made of GaN with a layer thickness of 5 nm was grown as thelast barrier layer 41.

Furthermore, on this barrier layer 41, an AlGaN layer with an Alcomposition ratio of 0.003 and a layer thickness of 5 nm was grown.Subsequently, a Mg-doped AlGaN layer 51 with an Al composition ratio of0.1 and a layer thickness of 5 nm, a Mg-doped p-type GaN layer 52 with alayer thickness of 80 nm (Mg concentration 2×10¹⁹/cm³), and a highlyMg-doped GaN layer 53 with a layer thickness of approximately 10 nm (Mgconcentration 1×10²¹/cm³) were stacked. Subsequently, the substrate 10with the aforementioned crystals grown thereon was taken out of thereaction furnace of the MOCVD apparatus.

Next, part of the above multilayer film structure was dry etched halfwaythrough the n-type GaN contact layer 22. Thus, the n-type GaN contactlayer 22 was exposed. An n-side electrode 70 made of Ti/Pt/Au was formedthereon. Furthermore, a transparent electrode 60 made of ITO (indium tinoxide) was formed on the surface of the highly Mg-doped GaN layer 53. Ap-side electrode 80 made of Ni/Au with a diameter of 80 μm, forinstance, was formed on part of the transparent electrode 60.

Thus, the samples x1-x3 were fabricated. The semiconductorlight-emitting devices of the samples x1-x3 thus fabricated are blueLEDs emitting at a main wavelength of 450 nm.

FIG. 3 is a graph illustrating an experimental result related to thesemiconductor light-emitting devices.

More specifically, FIG. 3 illustrates the variation of driving voltageVf for the samples x1-x3 with the layer thickness of the barrier layer41 varied. In FIG. 3, the horizontal axis represents the layer thicknesst_(b) (nm) of the barrier layer 41. The vertical axis represents thedriving voltage Vf of the semiconductor light-emitting device. Here, thedriving voltage Vf is expressed as a relative value, where the drivingvoltage is 1 when the layer thickness t_(b) of the barrier layer 41 is10 nm.

As shown in FIG. 3, the driving voltage Vf of the semiconductorlight-emitting device decreases with the decrease of the layer thicknesst_(b) (nm) of the barrier layer 41.

From the viewpoint of the driving voltage Vf with practical suitability,the layer thickness t_(b) of the barrier layer 41 is preferably 10 nm orless. More preferably, the layer thickness t_(b) of the barrier layer 41is 5 nm or less.

Second Experiment

In the second experiment, the ratio p/q, i.e., the ratio of the averageIn composition ratio p of the stacked body 30 to the average Incomposition ratio q of the light-emitting portion 40, was varied.

Specifically, as in the first experiment, the In composition ratio b ofthe barrier layer 41 was zero. The In composition ratio w of the welllayer 42 was 0.1. The layer thickness of the barrier layer 41 was 5 nm.The thickness of the well layer 42 was 2.5 nm.

As in the first experiment, the In composition ratio x of the firstlayer 31 of the stacked body 30 was zero, and its thickness was 1 nm.The number of stacked layers was 30 pairs. The thickness of the secondlayer 32 was 2.5 nm. The In composition ratio y of the second layer 32was varied as 0.04 and 0.08. Furthermore, a sample without the stackedbody 30 was fabricated. The sample without the stacked body wasdesignated as sample y1. The sample with the In composition ratio y ofthe second layer 32 being 0.04 was designated as sample y2. The samplewith the In composition ratio y of the second layer 32 being 0.08 wasdesignated as sample y3.

In the sample y1, p/q is zero. In the sample y2, p/q is 0.25. In thesample y3, p/q is 0.5.

The samples y1, y2, and y3 are near ultraviolet LEDs emitting at a mainwavelength of 400 nm.

FIGS. 4 and 5 are graphs illustrating an experimental result related tothe semiconductor light-emitting devices.

More specifically, FIG. 4 illustrates the variation of the drivingvoltage Vf of the semiconductor light-emitting device for the samplesy1, y2, and y3. In FIG. 4, the horizontal axis represents p/q, and thevertical axis represents the driving voltage Vf of the semiconductorlight-emitting device. Here, the vertical axis is expressed in relativevalue.

FIG. 5 illustrates the variation of the optical output power Po of thesemiconductor light-emitting device for the samples y1, y2, and y3. InFIG. 5, the horizontal axis represents p/q, and the vertical axisrepresents the optical output power Po of the semiconductorlight-emitting device in milliwatts (mW). Here, the vertical axis isexpressed in relative value.

As shown in FIG. 4, the driving voltage Vf of the semiconductorlight-emitting device decreases with the increase of p/q. Morespecifically, the decrease of driving voltage Vf is more significant forp/q over 0.4.

On the other hand, as shown in FIG. 5, the optical output power Po ofthe semiconductor light-emitting device increases with the increase ofp/q. More specifically, the optical output power Po significantlyincreases for p/q over 0.4.

From FIGS. 4 and 5, in view of both the decrease of driving voltage Vfand the increase of optical output power Po, it is found that high p/qis preferable. From the practical viewpoint, p/q is preferably higherthan 0.4.

Third Experiment

In the third experiment, the In composition ratio w of the well layer 42was set to 0.15, and the ratio p/q, i.e., the ratio of the average Incomposition ratio p of the stacked body 30 to the average In compositionratio q of the light-emitting portion 40, was varied.

Specifically, the In composition ratio b of the barrier layer 41 waszero. The layer thickness of the barrier layer 41 was 5 nm. On the otherhand, the In composition ratio w of the well layer 42 was 0.15. Thethickness of the well layer 42 was 2.5 nm.

Furthermore, the In composition ratio x of the first layer 31 of thestacked body 30 was zero. The thickness of the second layer 32 was 2.5nm.

Then, samples z with seven different values of p/q were fabricated byvarying the In composition ratio y of the second layer 32 or the layerthickness t_(x) of the first layer 31. Here, the thickness S_(all) ofthe stacked body 30 was adjusted to remain substantially constant(approximately 105 nm). The samples z are blue LEDs emitting at a mainwavelength of 450 nm.

FIGS. 6 and 7 are graphs illustrating an experimental result related tothe semiconductor light-emitting devices.

FIG. 6 illustrates the variation of the driving voltage Vf of thesemiconductor light-emitting device for the samples z. In FIG. 6 thehorizontal axis represents p/q, and the vertical axis represents thedriving voltage Vf of the semiconductor light-emitting device. Here, thevertical axis is expressed in relative value.

FIG. 7 illustrates the variation of the optical output power Po of thesemiconductor light-emitting device for the samples z. In this figure,the horizontal axis represents p/q, and the vertical axis represents theoptical output power Po of the semiconductor light-emitting device inmilliwatts (mW). Here, the vertical axis is expressed in relative value.

As shown in FIG. 6, also for near ultraviolet LEDs, the driving voltageVf of the semiconductor light-emitting device decreases with theincrease of p/q over 0.4.

On the other hand, as shown in FIG. 7, for the samples z, large opticaloutput power Po is generally maintained despite the variation of p/q.That is, good optical output is obtained for p/q higher than 0.4.

As described above, for small layer thickness t_(b) of the barrier layer41, the driving voltage Vf decreases. From the practical viewpoint, thelayer thickness t_(b) of the barrier layer 41 is preferably 10 nm orless. Furthermore, for high p/q, the driving voltage Vf decreases, andthe optical output power Po increases. In particular, for p/q higherthan 0.4, the decrease of driving voltage Vf and the increase of opticaloutput power Po are significant.

In a semiconductor light-emitting device based on nitridesemiconductors, the well layer 42 (quantum well layer) is made of InGaN.In such a semiconductor light-emitting device, the well layer 42including InGaN has a large lattice constant difference from thesubstrate used for crystal growth and various semiconductor layersstacked thereon (such as the GaN layer). Thus, lattice strain is likelyto occur in the well layer 42. Furthermore, the strain is augmented alsobetween the well layer 42 and the barrier layer 41 because of theuniform In composition ratio. If the well layers 42 and the barrierlayers 41 are stacked in a large number, this strain is accumulated to alarge extent. Thus, the lattice strain applied to the well layer 42 alsoincreases.

An excessive lattice strain applied to the well layer 42 is likely tocause defects due to lattice relaxation. Furthermore, if strain in thec-axis direction occurs in a hexagonal nitride semiconductor grown alongthe c-axis, the piezoelectric field modulates the band structure of theactive layer and leads to decreasing the light emission efficiency.

On the other hand, in the case where the thickness of the barrier layer41 is thinned to decrease the driving voltage, the crystallinity isdegraded. Simultaneously, the aforementioned strain applied to the welllayer 42 increases. This tends to further interfere with the increase oflight emission efficiency.

In contrast, from the results of the above first to third experiment,although the thickness of the barrier layer 41 is 10 nm or less, such as5 nm, if p/q is set higher than 0.4, then the optical output power Pocan be increased while decreasing the driving voltage Vf.

More specifically, the stacked body 30 is interposed between thelight-emitting portion 40 and the n-type semiconductor layer 20. Thestacked body average In composition ratio p of the stacked body 30 ismade higher than 0.4 times the light-emitting portion average Incomposition ratio q of the light-emitting portion 40. This relaxes thestrain applied to the light-emitting portion 40, and can sufficientlyimprove the crystallinity. Thus, even in the case where the layerthickness of the barrier layer 41 is thinned, the sufficientcrystallinity of the light-emitting portion 40 makes it possible toachieve the decrease of driving voltage while increasing the lightemission efficiency.

It should be noted that no one has ever paid attention to the average Incomposition ratios of the light-emitting portion 40 and the stacked body30. In the process of analyzing the above experimental results, theratio (p/q) of the average In composition ratios of the light-emittingportion 40 and the stacked body 30 has been noticed. This leads to thediscovery of the technique which can simultaneously achieve the decreaseof driving voltage Vf and the increase of optical output power Po.

Typically, in the stacked body 30 including nitride semiconductors, forinstance, the first layer 31 has an In composition ratio x of 0.03 and alayer thickness t_(x) of 2.5 nm. The second layer 32 has an Incomposition ratio y of zero and a layer thickness t_(y) of 2.5 nm. Thethickness S_(all) of the stacked body 30 is e.g. 50 nm. In thelight-emitting portion 40, for instance, the barrier layer 41 has an Incomposition ratio b of zero and a layer thickness t_(b) of 20 nm. Thewell layer 42 has an In composition ratio w of 0.4 and a layer thicknesst_(w) of 3 nm. The thickness T_(all) of the light-emitting portion 40 ise.g. 112 nm.

In this case, p/q is 0.34. In this case, as described with reference toFIGS. 4 to 7, the effect of decreasing the driving voltage Vf andincreasing the optical output power Po is lower than for p/q higher than0.4. Under this condition, S_(all)/T_(all) is 0.45.

Fourth Experiment

In the fourth experiment, the In composition ratio w of the well layer42 was set to 0.15, and the ratio of the thickness S_(all) of thestacked body 30 to the thickness T_(all) of the light-emitting portion40 was varied.

Specifically, the In composition ratio b of the barrier layer 41 waszero. The layer thickness of the barrier layer 41 was 10 nm. On theother hand, the In composition ratio w of the well layer 42 was 0.15.The thickness of the well layer 42 was 2.5 nm.

Furthermore, the In composition ratio x of the first layer 31 of thestacked body 30 was zero, and its thickness was 1 nm. The In compositionratio y of the second layer 32 was 0.08, and its thickness was 2.5 nm.

Then, by fixing the number of stacked well layers 42 and varying thenumber of stacked layers in the stacked body 30, samples x11, x12, andx13 were fabricated with three different values of the ratio(R=S_(all)/T_(all)) of the thickness S_(all) of the stacked body 30 tothe thickness T_(all) of the light-emitting portion 40. The samples x11,x12, and x13 are blue LEDs emitting at a main wavelength of 450 nm.

More specifically, the sample x11 has a layer thickness ratioS_(all)/T_(all) of 0.7. The sample x12 has a layer thickness ratioS_(all)/T_(all) of 1.1. The sample x13 has a layer thickness ratioS_(all)/T_(all) of 1.5.

FIG. 8 is a graph illustrating an experimental result related to thesemiconductor light-emitting devices.

FIG. 8 illustrates the variation of the driving voltage Vf of thesemiconductor light-emitting device for the samples x11, x12, and x13.In this figure, the horizontal axis represents S_(all) (nm)/T_(all)(nm), and the vertical axis represents the driving voltage Vf of thesemiconductor light-emitting device. Here, the vertical axis representsa relative value, where the driving voltage Vf is 1 when S_(all)/T_(all)is 1.1.

As shown in FIG. 8, when the layer thickness ratio S_(all)/T_(all) is 1or more, i.e., when the thickness S_(all) of the stacked body 30 isequal to or greater than the thickness T_(all) of the light-emittingportion 40, the driving voltage Vf of the semiconductor light-emittingdevice sharply decreases.

Practical Example

The semiconductor light-emitting device according to a practical examplehas the configuration of the semiconductor light-emitting device 110illustrated in FIGS. 1 and 2.

The In composition ratio b of the barrier layer 41 was zero. The layerthickness of the barrier layer 41 was 5 nm. On the other hand, the Incomposition ratio w of the well layer 42 was 0.1. The layer thickness ofthe well layer 42 was 2.5 nm.

The In composition ratio x of the first layer 31 of the stacked body 30was zero, and its thickness was 1 nm. The In composition ratio y of thesecond layer 32 was 0.08, and its thickness was 2.5 nm.

The number of stacked well layers 42 is eight pairs (the number of welllayers 42 is eight, and the number of barrier layers 41 is eight). Thenumber of layers in the stacked body 30 is 30 (the number of firstlayers 31 is 30, and the number of second layers 32 is 30).

In the semiconductor light-emitting device of this practical example,the average In composition ratio q of the light-emitting portion 40 isq=0.032. The average In composition ratio p of the stacked body 30 isp=0.02. Hence, p/q is 0.63. The thickness T_(all) of the light-emittingportion 40 is T_(all)=60 nm (except the thickness of the last barrierlayer 41). The overall thickness S_(all) of the stacked body 30 isS_(all)=100 nm. Hence, S_(all)/T_(all) is 1.6. This semiconductorlight-emitting device was measured by a total luminous flux measurementsystem. Then, the light emission wavelength was 407 nm, the drivingvoltage was 3.2 volts (V), and the optical output power was 17 mW. Thewall plug efficiency was 26%.

Comparative Example

The semiconductor light-emitting device of a comparative example was thesame as that of the above practical example except that the Incomposition ratio y of the second layer 32 of the stacked body 30 was0.04. In the semiconductor light-emitting device of the comparativeexample, p/q is 0.32. Furthermore, S_(all)/T_(all) is 1.6. In thesemiconductor light-emitting device of the comparative example, thedriving voltage was 3.3 V, and the optical output power was 14 mW. Thewall plug efficiency was 21%.

Thus, by setting p/q higher than 0.4, the decrease of driving voltage Vfand the increase of optical output power can be simultaneously achieved.

In the following, an example configuration of the semiconductorlight-emitting device 110 is described.

In the light-emitting portion 40, for instance, well layers 42 with alayer thickness t_(w) (nm) and barrier layers 41 with a layer thicknesst_(b) (nm) are alternately stacked.

The thickness of the barrier layer 41 is e.g. 5 nm or more and 10 nm orless. The thickness of the well layer 42 is e.g. 2 nm or more and 3 nmor less. Such barrier layers 41 and well layers 42 are stacked in arepeating structure of 6-8 periods. The barrier layer 41 is made of e.g.GaN. For blue LEDs emitting at a wavelength of 450 nm, the well layer 42is made of InGaN with an In composition ratio w of approximately 0.15.For near ultraviolet LEDs emitting at a wavelength of 400 nm, the welllayer 42 is made of InGaN with an In composition ratio w ofapproximately 0.1.

The thickness T_(all) of the light-emitting portion 40 having such astructure (except the last barrier layer 41) is e.g. 50 nm or more and110 nm or less. For blue LEDs, the light-emitting portion average Incomposition ratio q of the light-emitting portion 40 is set toapproximately 0.035 or more and 0.056 or less. For near ultravioletLEDs, the light-emitting portion average In composition ratio q of thelight-emitting portion 40 is set to approximately 0.023 or more and0.038 or less. This provides the desired wavelength of emission light,the desired low driving voltage, and high light emission efficiency.

In the stacked body 30, for instance, first layers 31 with a layerthickness t_(x) (nm) and second layers 32 with a layer thickness t_(y)(nm) are alternately stacked.

As described above, in the semiconductor light-emitting device accordingto this embodiment, the stacked body average In composition ratio p ofthe stacked body 30 is set higher than 0.4 times the light-emittingportion average In composition ratio q of the light-emitting portion 40.

Furthermore, the thickness S_(all) (nm) of the stacked body 30 is madeequal to or greater than the thickness T_(all) (nm) of thelight-emitting portion 40.

As described above, for blue LEDs, the light-emitting portion average Incomposition ratio q is e.g. 0.035 or more and 0.056 or less. The stackedbody average In composition ratio p is set higher than 0.4 times thisvalue of the light-emitting portion average In composition ratio q.

For near ultraviolet LEDs, the light-emitting portion average Incomposition ratio q is e.g. approximately 0.023 or more and 0.038 orless. The stacked body average In composition ratio p is set higher than0.4 times this value of the light-emitting portion average Incomposition ratio q.

From the viewpoint of crystallinity, the In composition ratio x of thefirst layer 31 (In_(x)Ga_(1-x)N) included in the stacked body 30 ispreferably in the range of zero or more and less than 0.2. Morepreferably, for instance, the In composition ratio x is zero.

The In composition ratio y of the second layer 32 (In_(y)Ga_(1-y)N) ispreferably in the range of greater than zero and less than 0.2 (where yis greater than x). More preferably, for instance, the In compositionratio y is in the range of 0.08 or more and less than 0.15.

The layer thickness t_(x) of the first layer 31 is preferably thickerthan 1 nm. More preferably, for instance, the layer thickness t_(x) isin the range of thicker than 1 nm and thinner than 3 nm.

The layer thickness t_(y) of the second layer 32 is preferably thickerthan 0 nm and thinner than 2 nm. More preferably, for instance, thelayer thickness t_(y) is in the range of 1 nm or more and 1.5 nm orless.

The inventor has confirmed that setting the number of pairs of the firstlayer 31 and the second layer 32 to 30 pairs or more is effective forhigh optical output power and low driving voltage in the semiconductorlight-emitting device. Here, setting the number of pairs to 30 pairs ormore is consistent with the thickness S_(all) (nm) of the stacked body30 being equal to or greater than the thickness T_(all) (nm) of thelight-emitting portion 40.

In the semiconductor light-emitting device including the stacked body 30and the light-emitting portion 40 as described above, the lattice strainapplied to the light-emitting portion 40 can be sufficiently reduced.Thus, the semiconductor light-emitting device achieves compatibilitybetween high light emission efficiency and low driving voltage.

Furthermore, in this embodiment, the thickness S_(all) (nm) of thestacked body 30 is equal to or greater than the thickness T_(all) (nm)of the light-emitting portion 40.

Thus, the distortion stress applied to the light-emitting portion 40 canbe sufficiently relaxed by the stacked body 30. Hence, the increase oflight emission efficiency and the decrease of driving voltage areachieved.

In the above example, the light-emitting portion 40 has an MQWconfiguration with barrier layers 41 and well layers 42 alternatelyrepeated therein. However, the light-emitting portion 40 may have an SQW(single quantum well) configuration with a well layer 42 sandwichedbetween a pair of barrier layers 41.

In the stacked body 30 with a plurality of first layers 31 and aplurality of second layers 32 alternately stacked therein, the averageIn composition ratio p may be determined either from any one pair of thefirst layer 31 and the second layer 32, or from all the first layers 31and the second layers 32.

In the light-emitting portion 40 with well layers 42 provided between aplurality of barrier layers 41, the average In composition ratio q maybe determined either from any one pair of the barrier layer 41 and thewell layer 42, or from all the barrier layers 41 and the well layers 42.

In the semiconductor light-emitting device according to this embodiment,it is only necessary that the average In composition ratio p determinedfrom one of the foregoing be higher than 0.4 times the average Incomposition ratio q determined from one of the foregoing.

Here, suppose that the stacked body 30 includes M (M being an integer of2 or more) first layers 31 and M second layers 32. In this case, asviewed from the n-type semiconductor layer 20, the j-th (j being aninteger of 1 or more and M or less) first layer 31 is denoted by “firstlayer 31 _(j)”. Furthermore, as viewed from the n-type semiconductorlayer 20, the j-th second layer 32 is denoted by “second layer 32 _(j)”.It is assumed that the first layer 31 _(j) is adjacent to the secondlayer 32 _(j) on the n-type semiconductor layer 20 side of the secondlayer 32 _(j).

In this notation, the first layer 31, includes In_(xj)Ga_(1-xj)N(0≦xj<1) and has a layer thickness t_(xj). The second layer 32 _(j)includes In_(yj)Ga_(1-yj)N (0<yj≦1) and has a layer thickness t_(yj).

Here, the average In composition ratio p of the stacked body 30 can bedetermined from the first layer 31; and the second layer 32; forarbitrary j.

More specifically, the average In composition ratio p of the stackedbody 30 in this case is determined as(xj×t_(xj)+yj×t_(yj))/(t_(xj)+t_(yj)). The average In composition ratiop thus determined is designated as p(j).

Furthermore, the average In composition ratio p of the stacked body 30can be determined from the M first layers 31 ₁-31 _(M) and second layers32 ₁-32 _(M).

More specifically, the average In composition ratio p of the stackedbody 30 in this case is determined asΣ(xj×t_(xj)+yj×t_(yj))/Σ(t_(xj)+t_(yj)). Here, Σ represents summationfor j=1, . . . , M. The average In composition ratio p thus determinedis designated as p(Σ).

On the other hand, suppose that the light-emitting portion 40 includesN(N being an integer of 2 or more) barrier layers 41 and N well layers42. In this case, as viewed from the n-type semiconductor layer 20, thei-th (i being an integer of 1 or more and N or less) barrier layer 41 isdenoted by “barrier layer 41 _(i)”. Furthermore, as viewed from then-type semiconductor layer 20, the i-th well layer 42 is denoted by“well layer 42 _(i)”. It is assumed that the barrier layer 41 _(i) isadjacent to the well layer 42 _(i) on the n-type semiconductor layer 20side of the well layer 42 _(i). Here, the last barrier layer 41, i.e.,barrier layer 41 _(N+1), is provided between the p-type semiconductorlayer 50 and the well layer 42 _(N) nearest to the p-type semiconductorlayer 50.

In this notation, the barrier layer 41, includes In_(bi)Ga_(1-bi)N(0≦bi<1) and has a layer thickness t_(bi). The well layer 42 _(i)includes In_(wi)Ga_(1-wi)N (0<wi<1) and has a layer thickness t_(wi).

Here, the average In composition ratio q of the light-emitting portion40 can be determined from the barrier layer 41 _(i) and the well layer42 _(i) for arbitrary i.

More specifically, the average In composition ratio q of thelight-emitting portion 40 in this case is determined as(wi×t_(wi)+bi×t_(bi))/(t_(wi)+t_(bi)). The average In composition ratioq thus determined is designated as q(i).

Furthermore, the average In composition ratio q of the light-emittingportion 40 can be determined from the N barrier layers 41 ₁-41 _(N) andwell layers 42 ₁-42 _(N).

More specifically, the average In composition ratio q of thelight-emitting portion 40 in this case is determined asΣ(wi×t_(wi)+bi×t_(bi))/Σ(t_(wi)+t_(bi)). Here, Z represents summationfor i=1, . . . , N. The average In composition ratio q thus determinedis designated as q(Σ).

In the semiconductor light-emitting device according to this embodiment,it is only necessary that one of the average In composition ratios p(i)and p(Σ) be higher than 0.4 times one of the average In compositionratios q(i) and q(Σ).

Second Embodiment

The second embodiment relates to a method for manufacturing asemiconductor light-emitting device.

FIG. 9 is a flow chart illustrating a method for manufacturing asemiconductor light-emitting device according to the second embodiment.

As shown in FIG. 9, this manufacturing method includes the followingforming steps.

This manufacturing method comprises the step of forming an n-typesemiconductor layer 20 including a nitride semiconductor on a substrate10 (step S110), the step of forming a stacked body 30 including a firstlayer 31 and a second layer 32 on the n-type semiconductor layer 20(step S120), the step of forming a light-emitting portion 40 including abarrier layer 41 and a well layer 42 on the stacked body 30 (step S130),and the step of forming a p-type semiconductor layer 50 on thelight-emitting portion 40 (step S140).

The step of forming the stacked body 30 includes the step of forming thefirst layer 31 including In_(x)Ga_(1-x)N (0≦x<1) with a thickness oflayer thickness t_(x) nanometers on the n-type semiconductor layer 20,and the step of forming the second layer 32 including In_(y)Ga_(1-y)N(0<y<1, x<y) with a thickness of layer thickness t_(y) nanometers on thefirst layer 31.

The step of forming the light-emitting portion 40 includes the step offorming the barrier layer 41 including In_(b)Ga_(1-b)N (0≦b<1, b<w) witha layer thickness t_(b) nanometers having a value of 10 nanometers orless on the stacked body 30, and the step of forming the well layer 42including In_(w)Ga_(1-w)N (0<w<1, y<w) with a thickness of layerthickness t_(w) nanometers on the barrier layer 41.

At least one of the step of forming the stacked body 30 and the step offorming the light-emitting portion 40 is performed so that the averageIn composition ratio of the stacked body 30 is higher than 0.4 times theaverage In composition ratio of the light-emitting portion 40, where theaverage In composition ratio of the stacked body 30 (stacked bodyaverage In composition ratio p) is (x×t_(x)+y×t_(y))/(t_(x)+t_(y)), andthe average In composition ratio of the light-emitting portion 40(light-emitting portion average In composition ratio q) is(w×t_(w)+b×t_(b))/(t_(w)+t_(b)).

This makes it possible to manufacture a semiconductor light-emittingdevice with increased light emission efficiency and reduced drivingvoltage. Furthermore, high wall plug efficiency can be achieved.

In the foregoing description, the film formation process isillustratively based on MOCVD (metal organic chemical vapor deposition).However, other methods are also applicable, such as molecular beamepitaxy (MBE) and halide vapor phase epitaxy (HVPE).

The “nitride semiconductor” referred to herein includes semiconductorshaving any composition represented by the chemical formulaB_(x)In_(y)Al_(z)Ga_(1-x-y-z)N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z≦1) where thecomposition ratios x, y, and z are varied in the respective ranges.Furthermore, the “nitride semiconductor” also includes those representedby the above chemical formula and further containing any group V elementother than N (nitrogen), further containing any of various elementsadded for controlling various material properties such as conductivitytype, and further containing any of various unintended elements.

The embodiments of the invention have been described with reference toexamples. However, the invention is not limited to these examples. Forinstance, various specific configurations of the components such as then-type semiconductor layer, p-type semiconductor layer, active layer,well layer, barrier layer, electrode, substrate, and buffer layerincluded in the semiconductor light-emitting device can be variouslymodified in shape, size, material, and layout by those skilled in theart. Such modifications are also encompassed within the scope of theinvention as long as those skilled in the art can similarly practice theinvention and achieve similar effects by suitably selecting suchconfigurations from conventionally known ones.

Furthermore, any two or more components of the examples can be combinedwith each other as long as technically feasible. Such combinations arealso encompassed within the scope of the invention as long as they fallwithin the spirit of the invention.

Furthermore, those skilled in the art can suitably modify and implementthe semiconductor light-emitting device described above in theembodiments of the invention. All the semiconductor light-emittingdevices thus modified are also encompassed within the scope of theinvention as long as they fall within the spirit of the invention.

Furthermore, those skilled in the art can conceive various modificationsand variations within the spirit of the invention. It is understood thatsuch modifications and variations are also encompassed within the scopeof the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

1. A semiconductor light-emitting device comprising: an n-typesemiconductor layer including a nitride semiconductor; a p-typesemiconductor layer including a nitride semiconductor; a light-emittingportion provided between the n-type semiconductor layer and the p-typesemiconductor layer and including: a barrier layer includingIn_(b)Ga_(1-b)N (0≦b<1) and having a layer thickness t_(b) (nanometers);and a well layer stacked with the barrier layer, includingIn_(w)Ga_(1-w)N (0≦w≦1, b<w), and having a layer thickness t_(w)(nanometers); and a stacked body provided between the light-emittingportion and the n-type semiconductor layer and including: a first layerincluding In_(x)Ga_(1-x)N (0≦x<1) and having a layer thickness t_(x)(nanometers); and a second layer stacked with the first layer, includingIn_(y)Ga_(1-y)N (0<y<1, x<y<w), and having a layer thickness t_(y)(nanometers), average In composition ratio of the stacked body beinghigher than 0.4 times average In composition ratio of the light-emittingportion, assuming that the average In composition ratio of thelight-emitting portion is (w×t_(w)+b×t_(b))/(t_(W)+t_(b)), and theaverage In composition ratio of the stacked body is(x×t_(x)+y×t_(y))/(t_(x)+t_(y)), and the layer thickness t_(b) of thebarrier layer being 10 nanometers or less.
 2. The device according toclaim 1, wherein a thickness of the stacked body is equal to or greaterthan a thickness of the light-emitting portion.
 3. The device accordingto claim 1, wherein the first layer is provided in a plurality, thesecond layer is provided in a plurality, and the plurality of firstlayers and the plurality of second layers are alternately stacked. 4.The device according to claim 1, wherein the barrier layer is providedin a plurality, and the plurality of barrier layers are stacked witheach other, and the well layer is provided in a plurality, and each ofthe plurality of well layers is located between the plurality of barrierlayers.
 5. The device according to claim 1, wherein the barrier layercontaining In has an In composition ratio lower than an In compositionratio of the well layer.
 6. The device according to claim 1, wherein thesecond layer has an In composition ratio higher than an In compositionratio of the first layer.
 7. The device according to claim 1, whereinthe second layer has an In composition ratio lower than an Incomposition ratio of the well layer.
 8. The device according to claim 1,wherein the first layer has an In composition ratio lower than an Incomposition ratio of the well layer.
 9. A semiconductor light-emittingdevice comprising: a substrate; an n-type semiconductor layer providedon the substrate and including a nitride semiconductor; a stacked bodyprovided on the n-type semiconductor layer and including a first layerand a second layer; a light-emitting portion provided on the stackedbody and including a barrier layer and a well layer; and a p-typesemiconductor layer provided on the light-emitting portion and includinga nitride semiconductor, the barrier layer including In_(b)Ga_(1-b)N(0≦b<1) and having a layer thickness t_(b) (nanometers), the well layerbeing stacked with the barrier layer, including In_(w)Ga_(1-w)N (0<w<1,b<w), and having a layer thickness t_(w) (nanometers), the first layerincluding In_(x)Ga_(1-x)N (0≦x<1) and having a layer thickness t_(x)(nanometers), the second layer being stacked with the first layer,including In_(y)Ga_(1-y)N (0<y<1, x<y<w), and having a layer thicknesst_(y) (nanometers), average In composition ratio of the stacked bodybeing higher than 0.4 times average In composition ratio of thelight-emitting portion, assuming that the average In composition ratioof the light-emitting portion is (w×t_(w)+b×t_(b))/(t_(W)+t_(b)), andthe average In composition ratio of the stacked body is(x×t_(x)+y×t_(y))/(t_(x)+t_(y)), and the layer thickness t_(b) of thebarrier layer being 10 nanometers or less.
 10. The device according toclaim 9, wherein a thickness of the stacked body is equal to or greaterthan a thickness of the light-emitting portion.
 11. The device accordingto claim 9, wherein the first layer is provided in a plurality, thesecond layer is provided in a plurality, and the plurality of firstlayers and the plurality of second layers are alternately stacked. 12.The device according to claim 9, wherein the barrier layer is providedin a plurality, and the plurality of barrier layers are stacked witheach other, and the well layer is provided in a plurality, and each ofthe plurality of well layers is located between the plurality of barrierlayers.
 13. A method for manufacturing a semiconductor light-emittingdevice, comprising: forming an n-type semiconductor layer including anitride semiconductor on a substrate; forming a stacked body including afirst layer and a second layer on the n-type semiconductor layer;forming a light-emitting portion including a barrier layer and a welllayer on the stacked body; and forming a p-type semiconductor layer onthe light-emitting portion, the forming the stacked body including:forming the first layer including In_(x)Ga_(1-x)N (0≦x<1) with athickness of layer thickness t_(x) nanometers on the n-typesemiconductor layer; and forming the second layer includingIn_(y)Ga_(1-y)N (0<y<1, x<y) with a thickness of layer thickness t_(y)nanometers on the first layer, the forming the light-emitting portionincluding: forming the barrier layer including In_(b)Ga_(1-b)N (0≦b<1,b<w) with a layer thickness t_(b) nanometers having a value of 10nanometers or less on the stacked body; and forming the well layerincluding In_(w)Ga_(1-w)N (0<w<1, y<w) with a thickness of layerthickness t_(w) nanometers on the barrier layer, at least one of theforming the stacked body and the forming the light-emitting portionbeing performed so that average In composition ratio of the stacked bodyis higher than 0.4 times average In composition ratio of thelight-emitting portion, assuming that the average In composition ratioof the stacked body is (x×t_(x)+y×t_(y))/(t_(x)+t_(y)), and the averageIn composition ratio of the light-emitting portion is(w×t_(w)+b×t_(b))/(t_(w)+t_(b)).
 14. The method according to claim 13,wherein a thickness of the stacked body is equal to or greater than athickness of the light-emitting portion.
 15. The method according toclaim 13, wherein the first layer is provided in a plurality, the secondlayer is provided in a plurality, and the plurality of first layers andthe plurality of second layers are alternately stacked.
 16. The methodaccording to claim 13, wherein the barrier layer is provided in aplurality, and the plurality of barrier layers are stacked with eachother, and the well layer is provided in a plurality, and each of theplurality of well layers is located between the plurality of barrierlayers.